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[MANILA] Coral reefs on which fish stocks have dropped below 300 kilograms per hectare are likely to collapse and lose their productivity, according to researchers.

They say that this figure provides an indicator of the 'tipping point' for sustainable fishing.

The researchers' conclusion, published in Proceedings of the National Academy of Sciences (PNAS) last month (11 October), is based on an analysis of the relationship of fish biomass to the health of coral reefs in more than 300 shallow coral reefs in nine countries of the Western Indian Ocean, including Kenya, the Maldives, the Seychelles and Tanzania.

"Fish and coral interact in a kind of synergistic relationship," said M. Aaron MacNeil, a research scientist at the Australian Institute of Marine Science, in Townsville, Australia, who helped to calculate the tipping point.

"The corals provide habitat for protection and settlement, as well as food for the fish, while the fish graze the algae that corals compete with for space, and cycle nutrients through the system."

Healthy coral reefs yield as much as 1,000 to 1,500 kilograms of fish per hectare. But they are vulnerable to factors such as overfishing and destructive fishing equipment that can eventually destroy them, allowing algae to take over.

"When fished biomass is below 300 kilograms per hectare, we see a steep and rapid decline in reef ecosystems," said MacNeil.

For reefs to remain productive, countries must introduce control measures against overfishing, and be able to regulate the number of fishers, fish catch, and fishing techniques, said Tim McClanahan, senior conservation biologist of the World Conservation Society, who co-designed the research and co-wrote the report.

McClanahan added that small meshed nets or traps should be banned on reefs, and limits placed on the number of fish that can be caught. Outright bans could also be instituted on catching some species.

Some see aquaculture as an alternative to open fishing on reefs and as an aid to for conservation.

But Nicholas Graham, senior research fellow of the Australian Research Council Centre of Excellence for Coral Reef Studies at James Cook University (JCU), and another member of the research team, pointed out that aquaculture is very complicated.

"It often has implications for the health of nearby habitats, or the need to catch large quantities of wild fish to feed the fish being cultured," said Graham.

Aside from regulating fisheries, marine protected areas (MPAs) can be set up where fishing is banned. These have been successful in the western Indian Ocean region and in Kenya, said the researchers.

"Setting up MPAs can effectively reduce the time needed to replenish coral reefs," said Porfirio Aliño, a Filipino marine scientist who has surveyed several MPAs in South-East Asia, and is lobbying lawmakers to build a network of MPAs in the Philippines.

MPA size should amount to at least ten per cent of a reef area to stop deterioration and biodiversity loss, said Aliño. It should be preferably co-managed by various stakeholders sharing management costs.

Vigorous legal enforcement and public education are also necessary to detect and control destructive and illegal fishing activities, Aliño recommended.

Link to full paper in PNAS

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